The invention general pertains to aircraft safety devices, and more particularly, to a system that prevents the build-up of flammable gas within an aircraft""s fuel tank ullage.
In recent years, the issue of jet fuel safety has gained significant momentum. Especially in the wake of the mid-air explosion of TWA 800 out of New York, which FAA experts recently concluded was due to flammable vapors which were emitted from unexpectedly heated Jet-fuel in the 747""s center fuel tank. The vapors were able to mix with a sufficient amount of oxygen in the fuel tank""s ullage (space above the fuel) to create a flammable mixture that was ignited by an unknown source. Consequently, the FAA has formed several Aviation Research Advisory Committee (ARAC) sub-committees comprised of experts in the fields of aircraft manufacturing, airport systems and fuel delivery/management to determine the best solutions for minimizing combustible conditions in jet fuel tanks.
The oxygen contained in ambient air is readily soluble in contemporary grades of commercial and military jet fuels. Absorbed oxygen and other volatile substances in such fuels have a tendency to degas from the fuels during the reduced atmospheric pressures of an aircraft""s ascent and cruise phases of flight. The degaussing oxygen increases existing oxygen levels in the aircraft""s fuel tank ullage, which can create a danger by broadening the flammable vapor range. For example, if a Boeing 737 center wing tank (CWT) having a capacity of 4000 gallons is filled with 3000 gallons of Jet-A fuel at sea level, the fuel can have as much as 15% ambient air absorbed therein with roughly 21% of the air being oxygen. As the aircraft ascends to a cruise altitude, the decreased atmospheric pressure in the CWT causes absorbed air to degas from the fuel (as predicted in Henry""s Law). At the decreased pressures, the degaussed 15% air will typically expand 300-400% (depending on altitude), which results in a significant addition of oxygen into the ullage of the CWT. When the oxygen level of the air in the ullage exceeds 8%, the fuel tank is considered to be on the threshold of a combustible condition. The petroleum""s volatile evaporated gas simply adds to this undesirable explosive condition.
Recent ARAC consultations have suggested that replacing ambient air, and the oxygen it contains, from jet fuel before it is pumped aboard aircraft would be desirable, if such a step can be achieved in a manner that is economically viable and would not increase refueling time. With the type of jet fuel delivery systems employed to date, it has not been practical to provide or deliver a very low-air content safety-enhanced jet fuel to aircraft refueling locations such as airports, airport fuel farms, airport terminals, air bases, aircraft-carriers, and the like. Nor has it been practical to maintain the very low-air content safety-enhanced feature of such fuels with those systems. The absence of such systems and the safety-enhanced jet fuels they would provide has lead to the consideration of supplying nitrogen gas to the fuel tank ullage in place of the air and oxygen. Covering the fuel surface and preventing fuel evaporation can enhance this improvement. A further improvement is to minimize mixing between nitrogen and incoming vent air. The concept further calls for isolating the incoming vent air and requiring that the fuel surface be covered as much as possible with a float.
Nitrogen (N2) production equipment must be provided in each aircraft to produce N2 on demand, and to do so during peak fuel use periods. The floats that cover the fuel surface will minimize the need for N2, but the wetting of the fuel tank walls will still create evaporative fuel conditions. This is greatest during maximum fuel use and is accumulative as the lowering fuel level creates a greater tank wall exposure. The present invention has features to minimize these conditions and maximize safer fuel tank ullage conditions.
A search of prior art patents and industry literature did not disclose any data that read directly on the claims of the instant invention.